1. Field of the Invention
This invention relates to methods of and apparatus for centralized toll restriction, wherein the restriction is accomplished in a telephone switching office. More particularly, this invention relates to methods of an apparatus for centralized toll restriction, wherein a plurality of PBX customers are serviced by centralized toll restriction equipment.
2. Technical Considerations and Prior Art
Generally, Private Branch Exchanges (PBXs) are connected to associated central offices by trunks, which allow making telephone calls outside of the PBXs, as may be required in the normal course of busninss. The completion of such calls is usually effected by automatic switching equipment, which can complete calls anywhere within the United States without operator assistance. However, in many instances, it is desirable for PBX customers to restrict the automatic completion of certain toll calls to a group of office codes and to complete other toll calls only with the assistance of an operator.
A common method of restricting such calls is to provide toll restriction equipment which monitors all calls directed outside of a PBX. Toll restriction equipment includes diverters which typically register the first three digits dialed on an outgoing call and compare them with a customer selected file of three-digit directory codes to determine if the call should be completed without operator intervention. If it is determined that operator assistance is not needed, the diverter disassociates itself from the call and allows the call to be completed. On the other hand, if it is determined that the registered digits are not included in the list of allowed codes, the diverter routes the call to an operator, and then disassociates itself from the call in order to service other PBX outgoing calls.
A major problem with the prior art approach is that each PBX is required to have individual toll restriction equipment, which according to the present state of the art is usually located at the customer premises. Changing a list of allowed directory codes in such equipment generally requires adding and removing a number of cross connections. As a result, changing the list of allowed directory codes, as well as normal maintenance, requires dispatching a telephone service technician to customer's premises. This generally consumes a large part of technicians's normal working day and also requires taking a toll restriction system out of service for a period of time, usually during business hours, when it is needed most.
To ease the maintanance problems, the prior art utilizes toll restriction equipment located in central offices. Since these offices often serve a large numbers of PBX customers, who must be furnished with individual call diverter equipment, such arrangements, generally, create space problems in urban central offices, which are usually short of equipment space when the toll restriction service is requested. Since each individual customer requires his own equipment, the space problem is aggravated. This approach is also inefficient, because generally, the individual toll restriction equipment is idle over a longer period of time than it is in use.
Consequently, prior art toll restrictors create maintenance problems when located on the PBX customer premises and create space and efficiency problems when located in central offices.
An attempt at obviating these problems is suggested in U.S. Pat. No. 3,736,383, in which a single call diverter serves a plurality of PBX customers, in order to restrict toll calls. However, with the approach disclosed in this patent, it is necessary to provide the central office controlling the PBXs with additional equipment which, of course, involves the concomitant problems of additional expense and interconnection interference with the existing system. In addition with the approach of that patent, it is not possible for separate phones connected through each PBS to be either restricted or not restricted, because the entire PBX is either restricted or not restricted.
A need, therefore, exists for a toll restriction approach for a plurality of PBXs, which minimizes all of the above-mentioned problems by providing an inexpensive and readily adaptable toll restriction system in the central office.